1. Field of the Invention
The present invention relates to a method of manufacturing a disk drive device and a disk drive device manufactured by the method, and in particular, to a method of manufacturing a disk drive device for reducing the adhesive amount of particles and a disk drive device manufactured by the method.
2. Description of the Related Art
Recently, the rotational accuracy of disk drive devices, such as HDDs and the like, has been dramatically improved by providing them with a fluid dynamic bearing unit. With this improvement, a disk drive device has been required to have a higher density and a more increased capacity. For example, a disk drive device magnetically storing data rotates a recording disk with recording tracks at high speed so that a read/write operation of data is executed while the flying magnetic head of the device is tracing the recording tracks with a slight flying height between both of them. In order to make such a disk drive device have a high density and an increased capacity, the width of the recording track is required to be narrow. As the width thereof becomes more narrow, the space between the magnetic head and the recording disk is required to be further narrowed. For example, the flying height between both is required to be extremely narrow, as narrow as 10 nm or less.
For the purpose of obtaining high density, magneto-resistance effect devices (hereinafter, referred to as MR devices) are in heavy use for the magnetic heads. On the other hand, because an MR device is used in an extremely-narrow flying height, the occurrence of thermal asperity failure (hereinafter, referred to as “TA failure”) or head crash failure can become a serious problem. TA failure is caused due to momentary heat generation in the MR device, the heat generation triggered by kinetic energy generated by the contact of the MR device with a minute foreign substance on the surface of the disk while the flying magnetic head traces the recording tracks. When the MR device is momentarily heated followed by being cooled, the resistance value of the MR device momentarily varies such that a reproduced signal is superimposed with a noise, causing the accuracy of reading the reproduced signal to be deteriorated. After intensive investigation, the present inventors have learned that TA failure is caused by foreign substances, which adhere to a disk drive device by being adhered to the surface of a recording disk through vibration, air flow, etc., having a size of approximately 0.1 μm to several μm (hereinafter, collectively referred to as “particles”).
The disk drive device is composed of an assembly, which includes: a base member; a bearing unit consisting of a sleeve, and a shaft relatively rotatable with respect to the sleeve; and a hub member rotatably supported, on the base member, by the bearing unit. The disk drive device is manufactured by mounting a recording disk on the hub member and by including a magnetic head, a drive device for the magnetic head, a control circuit, and other necessary parts.
Conventionally, machine parts of which a disk drive device is composed are assembled together after being treated with a cleaning process where foreign substances on the part such as dust are removed by cleaning with, for example, ultrasonic waves in a cleaning tank filled with a predetermined cleaning liquid. In the cleaning process, a so-called batch cleaning has been the mainstream, in which the parts to be cleaned are dipped into the cleaning liquid in a state where, for example, units of several hundreds of the parts to be cleaned are stacked together in a cleaning basket. Such a cleaning process is heavily used for cases where small parts are manufactured on a large scale. In the batch cleaning, the cleaning baskets and the parts to be cleaned are dried after cleaning and stored at a storage site such as a warehouse, as disclosed in Japanese Patent Application Publication No. Hei 7-124529. Thereafter, in an assembly process, the stored parts that have been cleaned are taken out and fed to an assembly line where the disk drive device is assembled.
In a manufacturing method using the aforementioned batch cleaning, the level of the cleanness as a whole by the cleaning process is generally low. For example, the cleanness level of a disk drive device is evaluated by the number of particles having a size greater than or equal to 0.5 μm per 1 cm2 (hereinafter, referred to as the “LPC”). The LPC is obtained in the following procedures: a device to be tested is dipped into a tank filled with, for example, 2000 cc of purified water; an ultrasonic wave of a frequency of 68 kHz and a power of 98 W are radiated onto the device to be tested for 120 seconds; and the number of particles existing in the purified water is counted with, for instance, a liquid particle counter such as CLS-700 or LS 200 (made by Particle Measuring Systems, Inc., U.S.A).
In the conventional batch cleaning, there has been a large variation in the cleanness levels of the parts to be cleaned between those that are placed in the outer area of the cleaning basket and those in the inner area thereof. Also, particles once detached from a part to be cleaned may likely to adhere to another part. Further, when intending to obtain a desired cleanness level, the cleaning takes a long time, causing the working efficiency to be deteriorated. Further, particles floating in air may sometimes adhere to a part that has been cleaned while it is being stored in the warehouse after the cleaning. As stated above, if many particles remain on the parts, TA failure is more likely to occur when the flying height of the magnetic head is small, becoming an obstacle for obtaining a higher density and an increased capacity of the disk drive device. Accordingly, it can be thought that a process of wiping the particles off with, for example, a solvent such as hexane, could be added before or after the assembly work; however, the addition of the process causes a decrease in the manufacturing efficiency, and, in many cases, the particles cannot be removed thoroughly with the wiping process.